Since polycarbonate is excellent in heat resistance, impact resistance and transparency, it has been widely used in many fields in recent years. Various studies have been carried out with processes for production of polycarbonate. Among them, polycarbonate derived from aromatic dihydroxy compounds such as 2,2-bis(4-hydroxyphenyl)propane, hereinafter “bisphenol A”, is industrialized by both processes of interfacial polymerization and melt polymerization.
According to the interfacial polymerization, polycarbonate is produced from bisphenol A and phosgene, but toxic phosgene has to be used. In addition, it remains a problem such as corrosion of equipments caused by by-products such as hydrogen chloride and sodium chloride and chlorine-containing compounds such as methylene chloride used in great quantities as a solvent, and difficulties in removal of impurities such as sodium chloride or residual methylene chloride which might have an influence on polymer properties.
Meanwhile, as a method for producing polycarbonate from an aromatic dihydroxy compound and diarylcarbonates, a melt-polymerization method has been long known, wherein, for example, bisphenol A and diphenylcarbonate are polymerized through a transesterification reaction under melting conditions while removing by-product aromatic monohydroxy compounds. Unlike the interfacial polymerization method, the melt-polymerization method has advantages such as not using solvents. However, it has an essential problem as follows: As the polymerization proceeds, viscosity of polymer in the system increases drastically to make it difficult to remove by-product aromatic monohydroxy compounds efficiently out of the system which would cause the reaction rate extremely decrease to make it difficult to increase the polymerization degree.
In order to solve the above problem, various attempts have been studied to extract aromatic monohydroxy compounds from polymer under conditions of high viscosity. For example, Patent Document 1 (Japanese Examined Patent Application Publication No. S50-19600) discloses a screw-type polymerization vessel having a vent. Patent Document 2 (Japanese Unexamined Patent Application Publication No. H02-153923) discloses a method using a thin-film evaporator in combination with a horizontal polymerizer.
Patent Document 3 (U.S. Pat. No. 5,521,275) discloses a method for redistribution of molecular weight of an aromatic polycarbonate under the presence of a catalyst using an extruder having a polymer seal and a vent under reduced pressure.
However, the methods disclosed in the above documents would not be able to increase the molecular weight of polycarbonate sufficiently. The above methods for increasing the molecular weight using catalyst in large quantity or using strict conditions such as applying a high shearing might cause problems which would have a significant influence to polymer such as the deterioration in hue or the progress of a cross-linking reaction.
It is known that the polymerization degree of polycarbonate can be increased by adding a polymerization accelerator to the reaction system of melt-polymerization. Increasing the molecular weight under a short reaction residence time and a low reaction temperature enables to increase the production volumes of polycarbonate which would make it easy to design simple and inexpensive reaction vessels.
Patent Document 4 (European Patent No. 0 595 608) discloses a method for reacting several diarylcarbonates at the time of redistribution which, however, would not bring a significant increase in molecular weight. Patent Document 5 (U.S. Pat. No. 5,696,222) discloses a method for producing a highly polymerized polycarbonate by adding a certain type of polymerization accelerator such as arylester compounds of carbonic acid and dicarboxylic acid including bis (2-methoxyphenyl) carbonate, bis (2-ethoxyphenyl) carbonate, bis (2-chlorophenyl) carbonate, bis (2-methoxyphenyl) terephthalate and bis (2-methoxyphenyl) adipate.
Said Patent Document 5 teaches that an ester bond is introduced by using esters as a polymerization accelerator, which causes the production of a polyestercarbonate copolymer, instead of producing a homopolymer, which is low in hydrolytic stability.
Patent Document 6 (Japanese Patent No. 4,112,979) discloses a method of reacting several salicylic carbonates with an aromatic polycarbonate in order to increase the molecular weight thereof.
Patent Document 7 (Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-514754) discloses a method of introducing polycarbonate oligomer and bis-salicylic carbonate or the like into an extruder to increase in molecular weight.
Patent Document 8 (Japanese Patent No. 4286914) discloses a method of increasing the amount of terminal hydroxy groups by an active hydrogen compound such as a dihydroxy compound and subsequently to carry out a coupling reaction of the aromatic polycarbonate having the increased amount of terminal hydroxy groups using a salicylic acid ester derivative.
However, the method disclosed in the above document requiring the amount of terminal hydroxy groups of polycarbonate is complicated in processes because it needs both a reaction process with an active hydrogen compound and a reaction process with a salicylic acid ester derivative. In addition, according to the method, polycarbonate having many terminal hydroxy groups is low in thermal stability and has a risk of deterioration in physical properties. As shown in Non-Patent Documents 1-2, the increase in the amount of hydroxy groups by active hydrogen compounds might induce a partial chain-decoupling reaction accompanied by widening of the molecular weight distribution. Furthermore, relatively large amount of catalyst is required to obtain a sufficiently high reaction rate, which might bring about deterioration in physical properties at the time of forming processes.
Several methods for producing polycarbonate by adding diol compounds in the reaction system are proposed. For example, Patent Document 9 (Japanese Examined Patent Application Publication No. H06-94501) discloses a process for producing a high-molecular polycarbonate by introducing 1,4-cyclohexanediol. According to the method disclosed therein, however, 1,4-cyclohexanediol is introduced together with an aromatic dihydroxy compound into the polycondensation reaction system from the beginning and therefore, 1,4-cyclohexanediol would be consumed first by the polycarbonate-forming reaction to form an oligomer, and then the aromatic dihydroxy compound would be reacted to participate the highly polymerization reaction. For this reason, it has such a defect that the reaction time would become relatively long, which might cause the deterioration of appearance features such as the color or hue.
Patent Document 10 (Japanese Unexamined Patent Application Publication No. 2009-102536) discloses a process for producing polycarbonate by copolymerizing specific aliphatic diol and etherdiol. However, since the polycarbonate disclosed therein has an isosorbide skeleton as a main structure, excellent impact resistance required to aromatic polycarbonates would not be exhibited.
A method of adding cyclic carbonate compound to the reaction system (Patent Document 11; Japanese Patent No. 3271353) and a method of adding a diol having hydroxyl groups having basicity higher than that of hydroxyl groups of the dihydroxy compound (Patent Document 12; Japanese Patent No. 3301453, Patent Document 13; Japanese Patent No. 3317555) are also provided. However, none of them succeeded in providing a highly polymerized polycarbonate resin having totally satisfying performances.
As mentioned above, the conventional methods for producing highly polymerized aromatic polycarbonate have many problems, and still there are requests for developing an improved production method which enables the increase in molecular weight of the aromatic polycarbonate resin satisfactorily while keeping good quality that a polycarbonate originally has.
A polycarbonate also has defects that it is poor in fluidity, and has difficulty in molding precision components or thin parts by injection molding. In order to improve fluidity, increase in a molding temperature and/or a die temperature is required, which might cause problems such as lengthening of a molding cycle, increase in mold cost and deterioration of polycarbonate during molding.
Examples of methods for improving fluidity include the decrease in weight average molecular weight of a polycarbonate. However, the polycarbonate thus obtained has defects of a significant decrease in impact resistance and stress cracking-resistance, and also in solvent resistance.
It is proposed to improve fluidity by widening a molecular weight distribution by blending polycarbonates having different molecular weight (Patent Document 14; U.S. Pat. No. 3,166,606, Patent Document 15; Japanese Unexamined Patent Application Publication No. S56-45945).
According to the above methods, polycarbonate resin compositions which are a non-Newtonian fluid having a large die swell were obtained. However, these polycarbonate resin compositions have lower fluidity under a low-shear stress compared to ones having a normal molecular weight distribution, whereas they have fluidity comparable with ones having a normal molecular weight distribution under a high-shear stress. These polycarbonate resin compositions certainly have a property of a non-Newtonian fluid having a large ratio of fluidity under a high-shear stress and those under a low-shear stress, but fluidity itself was not so excellent compared to the conventional ones. In addition, since the polycarbonate resin compositions have a wide molecular weight distribution, mechanical strength of the mold products would be deteriorated because of the low-molecular weight components. In the case of using a polycarbonate having an extremely high-molecular-weight range in order to obtain a polycarbonate having an intended molecular weight, the deterioration of hue of the molded products might be incurred caused by the increase in the content of colored components derived from a relatively long residence time.
The above-mentioned Patent Document 9 disclosing a method for producing a high-molecular-weight polycarbonate by introducing 1,4-cyclohexanediol does not teach anything about impact resistance or fluidity which are important properties of a polycarbonate, while it is described about heat resistance and tensile strength.
Besides the above documents, various methods for high fluidization of a polycarbonate are proposed. Examples of the proposals include Patent Documents 16-18 which disclose a method of adding a low-molecular weight oligomer to a polycarbonate or a method for high fluidization by determining the content of said oligomer and Patent Documents 19-20 which disclose a method for high fluidization by controlling production conditions.
Examples of the proposals also include Patent Documents 21-27 which disclose a method for high fluidization by adding other resins to a polycarbonate or copolymerizing, Patent Documents 28-30 which disclose a method for high fluidization by modifying the polymer molecular structure of a polycarbonate, Patent Documents 31-33 which disclose a method for high fluidization by modifying the end structure of a polycarbonate and additionally adding other resins and additives, Patent Documents 34-36 which disclose a method for high fluidization by devising an additive and Patent Documents 37-39 which disclose a fluidity-improving agent for a polycarbonate and a method for high fluidization by using said fluidity-improving agent.
While the above-mentioned methods may enable to achieve high fluidity, however, they have defects such as deterioration of properties that a polycarbonate originally has, complexity of the production process by adding a mixing and kneading operation or the like, deterioration of moldability such as releasability other than fluidity, restriction of intended use and possibility of having high toxicity. Therefore, it had not been easy to obtain a high-fluidity polycarbonate resin while keeping good physical properties such as impact resistance or heat resistance which are useful properties of an aromatic polycarbonate resin.
The present inventors had found a novel method of a chain extension by linking the end-capped terminal groups of an aromatic polycarbonate with an aliphatic diol compound to achieve a high polymerization rate to obtain an aromatic polycarbonate resin excellent in quality. See Patent Document 40 (WO2011/062220). According to the method, a highly polymerized aromatic polycarbonate resin having weight average molecular weight (Mw) of 30,000 to 100,000 can be produced in a short time by the chain extension by linking the capped end of an aromatic polycarbonate with an aliphatic diol compound. According to the method, since a polycarbonate can be produced by a high-rate polymerization reaction, branching and/or cross-linking reaction can be inhibited, and thus, deterioration of polymer such as color change can be prevented.
In addition, the present inventors had proposed a process for producing a branched aromatic polycarbonate resin having an intended branching degree comprising a process of transesterification reaction of an aromatic polycarbonate prepolymer introducing a branch structure with an aliphatic diol compound in the presence of a transesterification catalyst. See Patent Document 41 (PCT/JP2012/052988).
In addition, it is required that an aromatic polycarbonate compound which is a starting material or a prepolymer to be suitably used for producing a highly polymerized polycarbonate resin using an aliphatic diol compound has specific properties such as a certain concentration of terminal hydroxy groups.
Methods for reducing the concentration of terminal hydroxy groups of a polycarbonate prepolymer as the starting material for producing an aromatic polycarbonate resin are disclosed in Patent Document 42 wherein a catalyst combining basic nitrogen compounds and alkali metal or alkali earth metal compounds is used, Patent Document 43 wherein a specific ester compound is added, Patent Document 44 wherein an excess amount of aromatic diester carbonate is subjected to reaction, Patent Document 45 wherein conditions of the polycondensation process is selected or Patent Document 46 wherein terminal hydroxy groups are alkyl-etherified.
As mentioned above, the conventional methods for producing highly polymerized aromatic polycarbonate have many problems, and still there are requests for developing a polycarbonate resin having a satisfactorily high molecular weight while keeping good quality that a polycarbonate originally has, and an improved production method of highly polymerized polycarbonate resin.